A conventional plasma apparatus used for plasma cutting or plasma welding is shown in FIG. 4. A cross-section of the essential portion of the plasma arc torch of FIG. 4 is shown in FIG. 5. A plasma arc torch 1 for use with this plasma apparatus comprises an electrode 4, and a nozzle 5 made of copper and mounted so as to coaxially cover the electrode 4, wherein the electrode 4 and the nozzle 5 are electrically insulated from each other. In this electrode 4, an electrode element 2 having a high melting point is embedded in the tip of an electrode holder 3 made of copper or aluminum. Gas supply means 6 supplys a working gas between the electrode 4 and the nozzle 5, and a cooling water passage (not shown) for cooling the electrode 4 and the nozzle 5 is provided. Further, a high-frequency generating circuit 8 for causing insulation breakdown, and a DC power supply 9 for generating a main arc are connected to the plasma arc torch 1.
With such a construction, in operation, insulation breakdown A is first performed between the electrode 4 and the nozzle 5 by the high-frequency generating circuit 8. Next, the high-frequency generating circuit 8 is stopped, and a pilot current Ip is made to flow so that a pilot arc B is generated between the electrode 4 and the nozzle 5. When a main current Im is made to flow, electrical conduction can be obtained between the electrode 4 and a workpiece 7, and a main arc C is formed. Then, the pilot current Ip is shut off, and the main arc C is maintained between the electrode 4 and the workpiece 7. As a result, the workpiece 7 can be cut or welded satisfactorily.
However, in such a conventional plasma arc torch, when a plasma arc is generated and a workpiece is cut or welded, the electrode is consumed as a result of the generation of the plasma arc, and eventually the electrode will be unusable and have to be replaced. When the incidence of this replacement of this electrode is high, the operating cost is high, and many replacement operations must be performed, causing the operation efficiency to decrease.
In recent plasma cutting, there is an example in which oxygen is used as a working gas for cutting a soft steel plate. This example is widely used because the cutting speed and cutting quality of this type of cutting is higher than that of the plasma cutting in which an inert gas, such as nitrogen or argon, is used as a working gas. However, in a case where a gas containing oxygen is used as a working gas, the use of tungsten, which is a conventional electrode element material, is not practical since tungsten oxide has a low melting point and extremely low durability. To solve this problem, a method is known in which hafnium (Hf) is used as the electrode element, and an electrode having this material embedded in an electrode holder made of copper is used (refer to, for example, U.S. Pat. No. 3,597,649). However, even this electrode containing hafnium has a drawback in that the electrode is consumed earlier than an electrode containing tungsten which is used in an inert gas.
To solve this problem, several attempts have been made. One attempt is known in which the tip of an electrode and the inner and outer surface of a nozzle are electrically plated with nickel (Ni) or chromium (Cr) in order to prevent an arc from becoming unstable and to increase the lifetime of the electrode (refer to, for example, Japanese Patent Laid-Open No. 61-271800). Another attempt is known in which thermal conduction between the electrode element and the electrode holder made of copper is improved by disposing a spacer, formed of a gold or silver alloy, between the hafnium electrode element and the electrode holder made of copper, and the lifetime of the electrode is improved (refer to, for example, Japanese Patent Laid-Open No. 4-147772). However, the desired lifetime of the electrode cannot be achieved by these attempts. In particular, when the arc is generated and stopped frequently, the lifetime becomes extremely short.
In a cutting operation in which a conventional plasma arc torch is used, molten metal (dross) blows up when cutting starts. As a result of the deposition of this dross in the tip of the torch, the torch is likely to deteriorate or to be damaged. Furthermore, the tip of the torch is likely to conduct with a workpiece via the deposited dross, and an improper electric discharge, such as a double arc, occurs, causing the torch to be damaged. In addition, the electrode element for electric discharge is melted and damaged, a part of it is deposited in the nozzle or the like, causing a double arc to occur. Further, the melted electrode element for electric discharge causes the orifice hole in the tip of the nozzle to be deformed or blocked, thus causing a problem.